1. Field of the Invention
The present invention relates to an emission timing control apparatus for pulsed lasers, effecting pulsed laser emission by exciting a laser medium through pulsed discharge at a prescribed frequency of repetition using a magnetic pulse compression circuit, which improves the precision of synchronizing the emission timing of the pulsed laser with the control timing for the semiconductor exposure apparatus.
2. Description of the Related Art
Much attention is being given to the use of excimer lasers as exposure sources for reduced projection exposure apparatuses (referred to below as steppers) for semiconductor manufacturing. These are expected to provide a great many advantages: the possibility of extending the limits of the exposure light to below 0.5 xcexcm with the short wavelengths of excimer lasers (the wavelength for KrF is 248.4 nm); deeper focal depth than the g lines and i lines of the mercury lamps, which are usually used and which have the same resolution; increasing the exposure area with a small numerical aperture (NA) lens; and achieving high power.
FIG. 9 shows the general constitution of the control system for an excimer laser 1 and a stepper 10.
The excimer laser 1 comprises the following:
a laser chamber 2 housing discharge electrodes or the like;
a pulse power source apparatus 3 for applying the high frequency voltage, synchronized with the frequency of repetition of the pulsed discharge, between the discharge electrodes;
an energy monitor 4 to monitor the energy, wavelength, and the like of the laser beam output from the laser chamber 2; and
a laser controller 5 to supply and control laser gas, control laser oscillation wavelength, and control the power source voltage of the pulse power source apparatus 3, on the basis of the monitoring values from the energy monitor 4 and the energy command E from the stepper 10.
The stepper 10 comprises a movable wafer table 12 whereon wafers are mounted and a stepper controller 11 to transfer the pulse oscillation synchronizing signal TR, which is the trigger signal for repeated pulse oscillation, and the target energy command E for laser oscillation to the excimer laser. The wafers on the wafer table 12 are exposed with a reduced projection system using the laser beam from the excimer laser.
In recent years, magnetic pulse compression circuits have come to be used as the pulsed power source apparatus 3 in FIG. 9; these improve the durability of the main switch of a cyclotron, GTO, or the like. FIG. 10 shows an equivalent circuit for a capacitance switching, magnetic pulse compression discharge apparatus. FIG. 11 shows a waveform diagram of the voltage and current in each portion of the circuit in FIG. 10.
The discharge circuit in FIG. 10 is a two-stage magnetic pulse compression circuit utilizing the saturation phenomena of three magnetic switches AL0-AL2 comprising saturable reactors.
The energy command value E is input from the stepper 10 before the first laser oscillation trigger signal is received. The laser controller 5 therefore calculates the power source voltage necessary to output this energy and adjusts the voltage of the high voltage power source HV based on this calculated value. At this time, the capacitor C0 is precharged with a charge from the high voltage power source HV by means of the magnetic switch AL0 and coil L1.
Afterwards, the main switch SW is turned on when the first pulse oscillation synchronizing. signal (trigger signal) TR is received from the stepper 10 (FIG. 11, time t0). When the main switch SW is turned on, the potential VSW of the main switch abruptly drops to 0. After that, the time product (time integral value of voltage VC0) S0 of the voltage difference VC0xe2x88x92VSW of the main switch SW and the capacitor C0 reaches the limit value determined by the settings of the magnetic switch AL0. VC0 and VSW are the voltage of both terminals of the magnetic switch AL0. At that time t1, the magnetic switch AL0 becomes saturated and the current pulse i0 flows through the loop formed by the capacitor C0, magnetic switch AL0, main switch SW, and capacitor C1.
The time xcex40, from when that current pulse i0 starts to flow until it becomes zero (time t2), is determined by the inductance and capacitance of the capacitor C0, magnetic switch AL0, and capacitor C1, if loss due to the main switch SW or the like is ignored. More specifically, the charge transfer time 0 is the time necessary for charge to move completely from the capacitor C0 to the capacitor C1.
Meanwhile, the time product S1 of the voltage VC1 of the capacitor C1 reaches the limit value determined by the settings of the magnetic switch AL1. At this time t3, the magnetic switch AL1 becomes saturated and has low inductance. As a result, the current pulse i1 flows in the loop formed by the capacitor C1, capacitor C2, and magnetic switch AL1. This current pulse i1 becomes zero at time t4 once the prescribed transfer time 1, determined by the inductance and capacitance of the magnetic switch AL1 and capacitors C1, C2, has passed.
Also, the time product S2 of the voltage VC2 of the capacitor C2 reaches the limit value determined by the settings of the magnetic switch AL2. At this time t5, the magnetic switch AL2 becomes saturated, causing the current pulse i2 to flow through the loop formed by the capacitor C2, peaking capacitor CP, and magnetic switch AL2.
The voltage VCP of the peaking capacitor CP rises throughout the charging process. At the time t6 when this voltage VCP reaches the prescribed main discharge initiation voltage, the laser gas between the main electrodes 6 undergoes dielectric breakdown and the main discharge starts. The laser medium is excited by this main discharge and a laser beam is emitted after several nanoseconds.
This type of discharge action is performed repeatedly by the switching action of the main switch 5 synchronized with the trigger signal TR; as a result, pulsed laser oscillation is effected at the prescribed repetition frequency (pulse oscillation frequency).
The magnetic compression circuit shown in FIG. 10 is set so that the inductance of each stage of the charge transfer circuit, composed of magnetic switches and capacitors, progressively decreases in farther stages. Pulse compression is carried out so that the peak values of the current pulses i0 i2 gradually increase and the current amplitude gradually narrows. As a result, a strong discharge is attained between the main electrodes 6 in a short period of time. Also, each magnetic switch AL0-AL2 is reset at each pulse to the initial state with the reset circuit of a saturable reactor. The saturation point (action point) of each magnetic switch AL0-AL2 is the same for the voltage and becomes uniform from pulse to pulse.
With the abovementioned magnetic compression circuit, however, the saturation time xcex10 (xcex40+xcex11), (xcex41+xcex12) of each magnetic switch AL0-AL2 that is determined by the voltage time product changes when the initial charging voltage V0 changes. Accordingly the time td (referred to below as emission delay time) changes as well. The time td is from the time t0 when the trigger TR is input and the magnetic switch SW called up until the time t6 when the laser beam :is actually emitted.
In an excimer laser, as discussed above, the power source voltage V0 is one of control parameters for maintaining uniform laser output and can be varied during laser operation. Specifically, power source voltage V0 is variably controlled with consideration given to various factors such as power lock control for controlling power source voltage taking into consideration the drop in laser output due to a decrease in halogen gas, and spike-killer control for controlling power source voltage in order to resolve the spiking phenomenon wherein laser output becomes high in the spike zone, including the initial pulses of continuous pulse operation, compared to other zones.
In this way, the power source voltage V0 is one control parameter for an excimer laser; it is impossible to make the power source voltage uniform. The emission delay time td at each pulse oscillation is therefore varied according to the command voltage V0 at that time.
In a conventional system, the pulse oscillation synchronizing signal TR sent from the stepper 10 is used without further processing as a trigger signal for the main switch 5. In such a system, the emission delay time, from when the pulse oscillation synchronizing signal is generated until the laser beam is actually emitted, is different for each pulse. A problem is that it is therefore difficult to synchronize laser emission timing with stepper control timing in the stepper 10.
Especially in the case of the exposure system in the stepper 10 being a step and scan system, the stage (or laser beam) is moved during the exposure process. If the actual emission timing of each pulsed laser in the excimer laser is not completely synchronized with the timing for controlling the movement of the wafer (or laser beam) in the stepper, specifically if the pulsed laser beam is not emitted during the period while the stage is still, the stage moves during laser irradiation and the amount of exposure at each position varies greatly. For this reason, before now, the time from when the stepper outputs the pulse oscillation synchronizing signal TR until laser oscillation actually occurs was predicted based on experience and measurement data. The various types of control within the stepper were synchronized on the basis of this prediction.
Moreover, the step and scan system carries out the exposure process as the laser beam or wafer is shifted by a prescribed pitch xcex94P for the case where a laser beam, called a sheet beam, is shone on an integrated circuit chip 7 on a wafer as shown in FIG. 12. In this instance, exposure is made equal for all points on the IC chip 7 by setting the scanning pitch xcex94P and sheet beam radiation field so that the cumulative exposure (in FIG. 12, for example, the cumulative exposure for point A is P1+P2+P3+P4) for each position on the IC chip 7.
In the background art, the actual emission timing is predicted on the stepper side, but the actual emission timing varies depending on power source voltage and the like; thus, the predicted emission timing does not match the actual emission timing. A consequent problem is poor synchronization between laser emission timing and control timing on the stepper side.
The present invention was made in view of this situation; it is therefore an object of the present invention to provide an emission timing control apparatus for pulsed lasers, with more precise synchronization synchronicity between laser emission timing and control timing for the semiconductor exposure apparatus, so as to make uniform from pulse to pulse the time from when the pulse oscillation synchronizing signal is received until actual laser emission.
The invention corresponding to a first aspect of the invention relates to an emission timing control apparatus for a pulsed laser comprising: a magnetic pulse compression circuit including a multi-stage charge transfer circuit comprised of a plurality of magnetic switches connected serially to a charging power source and a plurality of capacitors connected parallel to the charging power source, for compressing current pulses in a plurality of stages using the multi-stage charge transfer circuit; switching means for carrying out a switching operation to connect and disconnect the charging power source to and from the magnetic pulse compression circuit; a laser discharge electrode connected to an output terminal of the magnetic pulse compression circuit; and control means for outputting a voltage command value to the charging power source, the pulsed laser executing pulsed laser oscillation at a prescribed repetition frequency by turning on the switching means with a trigger, being a pulse oscillation synchronizing signal having the prescribed repetition frequency received from a semiconductor exposure apparatus, wherein the emission timing control apparatus comprises: reference delay time setting means for setting a prescribed reference delay time in advance, the prescribed reference delay time being greater than a maximum value of a variable range of a real emission delay time from when the switching means is turned on until laser oscillation begins; delay time calculating means for calculating for each pulse oscillation a difference between the preset reference delay time and the real emission delay time for a pertinent pulse oscillation corresponding to the voltage command value output from the control means; and delay means for delaying a pulse oscillation synchronizing signal received from the semiconductor exposure apparatus by the time difference calculated in the delay time calculating means and outputting it to the switching means.
With the first aspect of the invention, the prescribed reference delay time is set in advance. The prescribed reference delay time is greater than the maximum value of the variable range of the real emission delay time from when the switching means is turned on until laser oscillation begins. As discussed below, for example, this reference delay time may be the time from when the abovementioned switching, means is turned on until laser oscillation begins, in the case of laser oscillation at a prescribed voltage value less than the minimum value for the voltage command value. Then, the difference between this reference delay time and the real emission delay time for the pertinent pulse oscillation is found for each pulse oscillation; the pulse oscillation synchronizing signal received from the semiconductor exposure device is delayed by this difference and output to the switching means. The time from when the laser oscillation pulse synchronizing signal is received until the laser beam is actually emitted is thereby caused to match the abovementioned established reference delay time for each pulse.
With the present invention, the time from reception of the laser oscillation pulse synchronizing signal until actual laser emission becomes uniform for each pulse. As a result, the laser emission timing can be completely synchronized with the control timing of the semiconductor exposure device without requiring the very difficult prediction control in the semiconductor device.
In the second aspect of the invention, the delay time calculating means in the first aspect of the invention finds the real emission delay time of the pertinent pulse oscillation according to an ambient temperature of the magnetic pulse compression circuit and the voltage command value output from the control means, and outputs a difference between the preset reference delay time and the real emission delay time of the pertinent pulse oscillation to the delaying means.
The second aspect of the invention compensates for the ambient temperature of the magnetic pulse compression circuit, as well as variations in power source voltage, and control the emission delay times so they are uniform. As a result, the precision to which the laser emission timing can be synchronized with the control timing for the semiconductor exposure device can be further improved.
In the third aspect of the invention, the upper limit value of voltage command value is a established so that a charge transfer time among the capacitors matches a saturation time of the magnetic switch.
The third aspect of the invention in sets the maximum value of power source voltage so that the charge transfer time among the capacitors matches the saturation time for the magnetic switches and controls power source voltage with a range such that this maximum voltage value is not exceeded. This invention therefore prevents the drop in current pulse peak value and the increase in current amplitude, as well as the situation where the magnetic switches become saturated during the charge transfer among capacitors.